Silicon semiconductor devices



Sept. 18, 1956 F. v. FROLA ETAL 2,763,322

SILICON SEMICONDUCTR DEVICES Filed may 1o, 195s swwmm jM/ ATTOR Y UnitedStates Patent O SILICON SEMICONDUCTOR DEVICES Frank V. Frola, TurtleCreek, and Milo W. Slye, Pittsburgh, Pa., assignors to WestinghouseElectric Corporation, East Pittsburgh, Pa., a corporation ofPennsylvania Application May 10, 1955, Serial No. 507,312

8 Claims. (Cl. 317-234) This invention relates to semiconductor devicesand in particular to silicon semiconductor rectitiers of the P-Njunction type which are specially adapted for power pur poses.

It has long been desirable to provide semiconductor devices comprising amember of silicon provided with at least one P-N junction. Whenalternating electrical current is applied to one side of the P-Njunction, rectification takes place since the junction has low impedanceto current ow from the P-type to the N-type areas but very highimpedance to current flow from the N-type to the ltype areas of thesilicon member.

The outstanding advantages of the silicon P-N junction material is thatit has a high rectifier eliciency at all temperatures up to about 220 C.Germanium rectifiers on the other hand become quite inethcient attemperatures approaching 100 C. As a consequence rectifiers preparedfrom germanium must be cooled with great care in order to prevent thetemperatures from exceeding a certain predetermined maximum, ordinarilyabout 80 C. High capacity Silicon diode rectifiers on the other hand canbe adequately air-cooled by conducting heat therefrom to simple fins orother radiator of moderate size. As a consequence, silicon rectifiersmay be safely employed under conditions where the ambient temperaturesare extremely high or where, because of the heavy loads, it would bedifficult to maintain temperatures of the rectiers below 100 C.

The preparation of P-N junction semiconductor devices from siliconrequires the solution of many difficult problems. The silicon materialitself must be employed in the form of extremely thin wafers whosethickness is of the order of l mils (0.010 inch). The silicon wafers arequite brittle and fragile so that they will break or shatter ifsubjected to any appreciable mechanical stresses. Breakage may beencountered not only during the manufacture and assembly of therectifiers but also during use by reason of differential thermalexpansion that takes place between the wafer of silicon and an endcontact to which it is aflixed, as the rectifier device embodying themheats up in use.

One of the critical problems in preparing satisfactory rectifiers fromsilicon semiconductor materials is to dissipate rapidly and efficientlythe heat developed during use'. While silicon has the ability to rectifyelectrical current at elevated temperatures of up to 220 C., the mostefficient rectification takes place at lower temperatures. Therefore,the lowest possible operating temperatures should be maintained.Excessive temperatures, beginning at about 220 C., may impair operationof the rectifier devices and even cause failure of the rectifier if itis subjected to heavy electrical loads while at such elevatedtemperatures. The silicon wafer must be mounted on an end contact of ahighly heat conducting metal such as molybdenum, and a solder must beemployed to assure good thermal and electrical contact. The term solderis used in the broadest sense to include brazing.

Other problems involved in producing satisfactory rec- 2,763,822Patented Sept. 18, 1956 ICC tifying devices relate to the protection ofthe silicon wafers from adverse atmospheres and contamination. Since forsilicon conductor applications the silicon should be of the highestorder of purity, ordinarily having less than one part by weightY ofimpurity per l0 million parts of silicon; moisture, small particles ofdirt, and the like settling on the silicon can react or diffuse into thesilicon wafer and result in damage or impairment of its eiciency inrectification.

The object. of this invention is to provide a semiconducting membercomprising silicon bonded to a heat absorbing and dissipating contactmember by means of a solder composed of silver alloyed with at least oneelement of group IV-B of the periodic table excluding carbon.

A furthur object of the invention is to provide a semiconductorrectifier device wherein silicon with a P-N junc tion is bonded by afused layer of an alloy composed of silver and` at least one elementfrom the group consisting of tin, silicon, germanium and lead, to anend' contact or support of molybdenum, tungsten or base alloys thereof.

A still further object is to provide an air-cooled power rectifiercomprising heat radiator meansV and a hermetically sealed casing withinwhich is an end Contact of molybdenum bonded tol a silicon wafer by asolder composed of siiver alloyed with at least one element of the groupconsisting of tin, silicon, germanium and lead.

Another object of the invention is to provide a process for producingunitary semiconductor rectifier devices by heating to a temperature ofbetween 850 C. to 925 C. a silicon wafer, a molybdenum or tungsten endcontact, and a fusible solder of an alloy composed of silver and atleast one element from the group consisting of tin, silicon, germanium,and lead.

For a better understanding of the nature and objects of the inventionattention is directed to the accompanying drawing, in which- Fig. 1 isavertical` cross-section through an assembly prior to fusion;

Fig. 2 is a vertical cross-section of a modied form of assembly;

Fig. 3 is a vertical cross-section through a vacuum fue nace suitablefor producing bondedsilicon rectifier elements; and

Fig. 4 is an enlarged vertical cross-section of a complete air-cooledrectier device embodying aV silicon P-N junction.l

Briey, We have discovered that it is possible to produce outstandingsemiconductor devices, and particularly P-N junction rectiiiers, bybonding silicon wafers by means of selected silver alloys to a heatdissipating and supporting end` contact of molybdenum, tungsten or basealloys thereof. The properly bonded silicon wafer is protected fromdamage over the widest ranges of temperature fluctuations, and heatdeveloped in the silicon wafer during use as' a rectifier' is conductedaway rapidly to the end contact by the solder.

A number of critical requirements must be met by a solder composition inorder to produce the best silicon rectifier diode units. In particular,the solder must have the following properties:

l. Wet and bond to silicon both while in the molten state and in thesolidified state.

2. Wet and bond to molybdenum, tungsten and base alloys thereof, bothwhile in the molten and solidified state.

3. H-ave low electrical and thermal resistance.

4. Have a suitable matching coetiicient of thermal expansion and goodductility which will enable the solder to unite a silicon wafer to amolybdenum end contact over a temperature range of 925 C. to -l00 C.without breaking` away from or damaging the silicon.

5. Will not contaminate, adversely react with or otherwise impair theproperties of the silicon wafer.

6. Low vapor pressure at elevated temperatures so that leakage paths arenot produced during soldering and other high temperature operations.

7. Require no ux to secure a good metal-to-metal bond.

In particular, we have discovered that highly satisfactory siliconsemiconductor devices may be prepared by bonding silicon to an endcontact, of molybdenum, for instance, by means of a silver soldercomposed of an alloy of silver and at least one element from group lV-Bof the periodic table selected from the group consisting of tin,silicon, germanium and lead. These solders satisfactorily meet all therequirements above set forth. The alloys are composed of at least 5%silver, the balance not exceeding 90% by weight of tin, not exceeding20% by weight of silicon, not exceeding 50% by weight of germanium andnot exceeding 95% by weight of lead. Particularly good results have beenobtained with binary alloys comprising silver and from 65% to 90% oftin; silver and from 5% to 16% by weight of silicon; silver and from 25%to 50% by weight of lead; and, silver and from 5% to 30% by weight ofgermanium. Ternary alloys of silver, tin and silicon; silver, lead andsilicon; and silver, germanium and silicon are particularlyadvantageous. For example, the ternary alloys may comprise 50% to 80%silver and 5% to 16% silicon, the balance being tin, lead or germanium.The silver may include small amounts of other elements and impurities,providing, however, that no significant amount of a group III element ispresent.

When these silver alloys are applied to the silicon wafer, some of thesilicon from the wafer dissolves in the alloy and, consequently, binaryand ternary alloys which are applied without silicon being presenttherein will, after fusion, contain a small but substantial amount ofsilicon. Thus, an alloy comprising 85% silver, 10% tin and 5% germaniumapplied to a silicon Wafer will, after fusion, contain from 5% to 16% byweight of silicon, depending upon the length of time and thetemperatures to which the solder alloy and the silicon are subjected.

We have secured excellent results with alloys comprising from 2% to 5%by weight of germanium, the balance, 98% to 95%, being silver. Thinsheets of these binary silver alloys have been applied to the siliconwafers and after heating the assembly to brazing ternperatures, thesilver alloy heats and bonds to the silicon, and a portion of thesilicon diffuses therein so that the fused bonding layer may comprisefrom 5% to 16% by weight of silicon, 1% to 4.5% by weight of germaniumand the balance being silver. The germanium-silver alloy is ductile andmay be readily rolled into thin films of a thickness of from l to 2mils. The thin lms may be then cut or punched into small pieces ofapproximately the same area as the silicon wafers and applied thereto.However, the alloy may be prepared in powder or granular form and a thinlayer thereof applied to the end contact either dry or in the form of apaste in a volatile solvent, such as ethyl alcohol.

Reference should be made to Fig. 1 of the drawing where there isillustrated an assembly l0 of the rectifier components previous to beingheated in a furnace to cause fusion and bonding of all of the componentsinto a unitary rectifier device. The assembly comprises an end contact12 which may be of a substantial thickness of the order of to 100 milsand from 1/4 to 2 inches in diameter, and even greater in the case oflarge rectiers. The end contact comprises La metal selected from thegroup containing molybdenum, tungsten or base alloys thereof. Bothmolybdenum and tungsten have a coeicient of linear thermal expansioncorresponding closely to that of single crystal, silicon (about 4.2)(10-per degree centigrade). Alloys of molybdenum and tungsten,

for example an alloy composed of 5% tungsten and 95% molybdenum, alsohave nearly the same coelcient of thermal expansion as silicon. Bothmolybdenum and tungsten can be alloyed with minor amounts of othermetals without greatly changing their coeflicient of thermal expansion.Thus, molybdenum may be alloyed with 5% to 25% by weight of a platinummetal, for example osmium or platinum, chromium, nickel, cobalt,silicon, copper and silver. A coefficient of thermal expansion ofbetween about 3.8 105 and 5)(10Ei per degree centigrade is satisfactoryfor cooperation with a silicon wafer. Molybdenum has given outstandingresults in practice. While both molybdenum and tungsten have excellentthermal conductivities so that they will carry away heat rapidly fromsilicon disposed in contact therewith, the molybdenum has a much lowerdensity and for many applications it will be found preferable. Thus, inequipment which is subject to motion, members of the lighter molybdenumwill `have lower inertia effects than a similar size member of tungsten.Hereinafter, molybdenum will be specifically referred to, but it will beunderstood that tungsten or an alloy of either tungsten or molybdenumcan be substituted therefor.

The molybdenum end contact 12 is carefully cleaned by abrading, etchingand washing or any one such as abrading with a sand blast, to remove allsurface contamination therefrom. In order to produce the best bonding,it has been found desirable to apply beforehand a thin coating 13 and 14of silver or of an alloy of silver to both of the face surfaces of thecontact 12. We have initially applied a coating 13 of silver solely tothe lower surface. A satisfactory method of applying the silver is tocoat the face surfaces with silver or an alloy comprising silver and 5%germanium, either in the form of a thin sheet or fine powder, andheating the molybdenum so treated in a vacuum or a hydrogen atmosphereat 1200 C. The silver will rapidly wet the surface of the molybdenum andspread thereover uniformly. In other instances, we have rst coated themolybdenum surfaces with a nickel phosphide coating following theprocedure set. forth in application Serial No. 301,016, assigned to thesame assignee as the instant application. A coating of the nickelphosphide is chemically deposited from an aqueous solution containing,for example, 0.02 mole/liter of nickel sulfate, 0.07 mole/liter ofNiClz, and 0.225 mole/liter of sodium hypophosphite upon simplyimmersing the molybdenum members therein. After the members have beenimmersed for a period of time of the order of tive minutes to 30 minutesthey may be removed from the solution, dried and then heated to atemperature of 1200 C. for one-quarter of an hour or longer. A thincoating of nickel phosphide comprising 95% or more nickel, will coverthe molybdenum surfaces, and it may then be silver plated in aconventional type of silver cyanide electroplating solution to applythereto approximately a coating of l mil thickness of silver 14.

There is then placed upon the silver coating I4 of the molybdenum endContact 12 a layer 16 of the silver alloy to function as a solderbetween the silicon wafer 18 and the end contact 12. We have employedwith considerable success films or foils of silver and silver alloys,the foils being of a thickness of from l to 2 mils, and being ofsubstantially the same area as a silicon wafer 18 place-il thereon.However, the silver or silver alloy may be applied in the form ofpowder, paste and the like with satisfactory results. The upper surfaceof thc end contact 12 is illustrated as being flat as is the lowersurface of thc silicon surface 18. However, it will be understood thatwhile at surfaced members are particularly convenient to prepare andemploy, other shapes may bc made and used. In all cases, it is necessarythat the meeting surfaces of the end contact and the silicon waferconform closely to one another so that a good silver alloy solder bondeventually result to provide for the best possible thermal conductivity.

The silicon wafer 18 will ordinarily be of a thickness of approximatelyl mils. Substantially greater thicknesses, such as 25 mils, forinstance, result inless effective rectier operation, while asubstantially thinner silicon wafer, below mils, for instance, may besubjected to striking through or otherwise failing. The silicon wafercomprises a single crystal and will have N-type conductivity. Thesilicon Wafer is prepared with finely polished or lapped surfaces whichare etched in a solvent, such as the HF-HNOa and mercury solution setforth in Patent 2,705,192 to remove any surface impurity, looseparticles, projections, roughness, and the like.

Upon the upper surface of the sil-icon wafer 18 there is placed a thinlayer 2i) comprising for example ai foil of a thickness of from l to 2mils of aluminum or an aluminum base alloy, and preferably an alloy ofaluminum with an element from group III or IV, such, for example, assilicon, gallium, indium and germanium, which functions not only toenable soldering or bonding of the silicon wafer to an upper contact 22,but also produces P-type conductivity by diffusion into the upperportion of the N-type silicon wafer. The layer 20 may comprise purealuminum with only slight amounts of impurities being present, such asmagnesium, sodium, zinc, and the' like,-

or an alloy composed of aluminum as a major component,

the balance being silicon, gallium, indium, and germanium individuallyor any two or all of the latter being present. These alloys should besolid up to about 300 C. Thus, a foil of 95% aluminum and 5% silicon;88.4% aluminum- 11.6% silicon; 90% aluminum- 10% germanium; 47%aluminum-53% germanium; 88% aluminum- 12% indium; 96% aluminum-4% byweight of indium; 50% aluminum-20% silicon-20% indium-10% germanium; 90%aluminum- 5% silicon5% indium; 85% aluminum- 5% silicon-5% indium-5%germanium; :and 88% aluminum- 5% silicon-2% indium-3% germanium and 2%indium may be employed (all parts being by weight). be substantiallysmaller than the area of the silicon wafer 18, and that it be centeredon the wafer 18 with a substantial clearance from the corners or edge ofwafer I8. It is not necessary that the aluminum layer 20 be a foil or aseparate layer. We have found itpossible tovapor coat aluminum or thealuminum base alloy in a vacuum upon the lower surface of an uppercontact 22. A-lternatively, the selected central portions of the uppersurface of the silicon wafer maybe vapor coated with aluminum oraluminum base alloy, by masking the edges of the wafer.

The upper contact 22 is preferably composed of the same metal as thelower contact; namely, molybdenum, tungsten or base alloys thereof. Theupper contact cornprises a at disc portion 24, which is smaller in areathanr the upper surface of the silicon wafer 18. The Contact 22compris-es an upwardly extending button 26 provided with a cup or well28 adapted to receive the end of aconductor. The upper contact 22 may bereadily prepared from molybdenum by machining. We have found it`desirable to coat only the Well 28 of the contact 22 with a thin coating29 of a Suitable solder, such as 70% silver- 30% gold alloy, 97%silver-3% germanium alloy, gold alone, or an alloy comprising 95% silverand 5% silicon. Care must bc observed to prevent any silver beingpresent at or near the edges of the disc portion 24 and aluminum layer20 to avoid a short-circuit connection being produced.

The upper contact 22 may be of a simpler construction than shown in Fig.l. Thus, round discs may be punched from a 30 mil to 50 mil thick sheetof molybdenum, then the round discs are counterbored to a depth of froml5 to 25 mils, to produce a cup or well which well is then coated with asolder, such as 95% silver-5% germanium alloy.

lt will be understood that the upper contact need not have a cup orwell, though such cup is advantageous for lt is critical that thealuminum layer 20 soldering of a conductor thereto; The upper contactcan be of any suitable shape or structure which will enable firm bondingof a conductor thereto as by soldering and will be satisfactory.

In some instances, we have been able to reduce the number of parts inthe rectier assembly in the manner shown in Fig. 2 of the drawing. Theassembly 40 in Fig. 2 comprises an end contact 42 of molybdenum on whichthere is applied to both the upper and lower surfaces a coating 43 and44 of the order of 2 mils thickness of silver or silver alloy. Suchsilver coating may comprise a foil of silver alloy applied to the bottomand upper sides of the molybdenum member 42 and the assembly introducedinto a furnace with a protective atmosphere of hydrogen or in a vacuumand heated to 1200 C. for l5 minutes in order to fuse the silverthereto. A suitable alloy for this coating 43-44 is one of silver and 5%germanium. The remainder of the assembly, namely, the silicon wafer 16,the aluminum layer 20, and the upper contact 22, are similar to thearrangement illustrated in Fig. l of the drawing.

The assembly of Fig. l or 2 is then placed within a furnace 50,illustrated in Fig` 3 of the drawing. The furnace comprises a base 52through which pass conduits 54 connected with a pump or other sourcecapable of producing a high vacuum and another conduit 56 forintroducing a protective gas, such a helium, argon, or the like, and forbreaking the vacuum which may be created in the furnace. The furnaceproper comprises a bell 58 of a heat resisting glass, such as, forexample, a 96% silicon dioxide glass, litting into a sealing gasket 60applied to the base 52. A refractory support 62 mounted on the base 52is adapted to suspend a graphite block 64 provided with one or morecavities 66 adapted to receive the assembly 10, such as shown in Fig. lof the drawing. A weight |58y of a high melting point non-reactive metalor other material, such as graphite, is applied upon the contact 22 ofthe assembly in order to apply a suitable light pressure to theassembly. An encircling heater 70 comprising a heating element 74disposed within an annular groove 72 is adapted to be lowered about thebell 58 in order to heat the graphite block 64 by radiating heatthereto- In practice, we have placed a number of assemblies 16 withinthe graphite block 64, placed the bellV 58 thereover in position in thegasket 60 and evacuated the space within the bell 58 through the conduit54. The pressure'within the bell is reduced to an extremely low value ofless than 0.01 micron. Heat is then radiated to the graphite block 64 byenergizing the resistance heating element 74. Usually heating causesevolution of gases and evacuation is continued throughout the operation.A thermocouple is placed within the depression 66 adjacent the assembly10 in order to determine the temperature present therein.

The maximum temperatures necessary for satisfactory bonding of theassembly 1'0 have been from 850 C. to 925 C. The aluminumv or aluminumalloy layer 20 will not properly wet silicon and molybdenum untiltemperatures of at least about 570 C. are attained, and 800 C. isusually required for best results. Particularly good results have beenobtained when the temperatures of the' furnace was controlled so thatassembly 1.0 reached a peark of from 870 C. to 900 C. Such peaktemperatures are held for a brief period of time, ordinarily not over aminute, and the temperature is then promptly reduced. No' particulardifferences have been found in rectifiers wherein the rate of heating,and the corresponding rate of cooling, was varied to such an extent thatthe ternperature rise from C. to 875 C. took place in as short a time as5 minutes or as long as 60 minutes. We have found that the silversolders of the present invention wet both silicon and bolybdenum rapidlyand dissolve a small amount of the silicon in a short while after theyreach their melting point; holding for any excessive times while thesilver alloy is fused does not produce any particularly beneficialresults.

The upper surface of the N-type silicon wafer 18 is wetted by the moltenaluminum layer 20 and the aluminum diffuses into the N-type siliconwafer, producing a Ptype layer contiguous with the aluminum which is ofclosely the same area as that of layer 20 at the upper surface.Therefore, a P-N junction results in the silicon.

The temperature required for the bonding of the silicon to themolybdenum end contact 12 is dependent on the fusion point of the silversolder 16. While some of the solders of the present invention have beenfound to melt as low as about 225 C., we prefer to employ solders whosemelting point is at least 400 C. and preferably of the order of 600 C.to 700 C. Wetting of the silicon does not occur below about 570 C., andusually occurs at about 800 C. In no event is it desirable to employ anysolder that requires a temperature of substantially over 925 C. to causefusion and bonding. At temperatures well above 950 C. a detrimentaleffect has been found to take place in the silicon so thatunsatisfactory rectifiers are produced.

After the assembly 10, or the assembly 40, has been subjected to heatingin the furnace to cause fusion of the silver base solders with bondingof the components into a unitary diode or rectifier member, theresulting diode members are placed in a hcrrnetically sealed metalcasing. The end contact 12 is soldered to the metal casing by coating 13to enable heat to be conducted to the casing. The metal casing isassociated with an ecient radiator dissipating heat to the atmosphere.If desired, the casing can be partly or completely filled with aninsulating dielectric liquid to assist heat dissipation. However, suchdielectric liquid is not necessary.

A particularly satisfactory complete air-cooled rectifier device is theunit shown in Fig. 4 of the drawing. The complete rectifier device 100comprises a body 102 of aluminum or copper or other suitable good heatconducting metal or alloy. The periphery of the body 102 is providedwith a plurality of fins for dissipating heat rapidly to the air. Thebody 102 comprises a well 106 within which is placed a closely fitting,hermetically sealed casing 108 which encloses the rectifier assembly 10.The casing 108 may be soldered to the walls of the well 106. A flexibleconductor of copper or silver soldered to well 28 in the upper contact22 extends upwardly to and is soldered within a cavity 112 of a bushing114. The bushing 114 comprises an electrically insulating ring 116 ofglass bonded to a lianged ring 118, of the nickel-ironchromium alloyknown as Kovar alloy, hermetically united by solder 120 to the wall ofthe metallic casing 108. A exible conductor 122 is attached to a cup 124disposed inthe exterior portion of the bushing 114. Alternating currentto be rectified is carried by the conductor 122 to bushing 114, thenceto the conductor 110, and to the upper part of the silicon wafer 18which has a P-N junction. Another current conductor is attached to thebody 102 by means not shown. Electrical current is carried by such otherconductor, the end contact l2 and the silicon wafer in circuit with eachother.

Rectifiers similar to those shown in Fig. 4 of the drawing have beenproduced and tested with exceptionally satisfactory results. They may beemployed at ambients of below 100 C. to about 220 C. Thus, with asilicon wafer of a diameter of one-quarter inch, we have been able torectify from to 30 amperes depending on the size of the fins 104 and theavailable cooling. By blowing air by means of a fan through circulartins 104 of a depth of about one inch. we have been able to rectify forshort periods of time of the order of one hour, 60 amperes of electricalcurrent at 100 volts through this same unit. In one instance, a siliconrectifier constructed as shown in Fig. 4 with a silicon wafer 5/a inchin diameter rectified 700 amperes of electrical current for a `briefperiod of time with fan cooling.

For a one-quarter inch diameter silicon diode, the forward drop wasbetween 0.8 to 1 volt at 30 amperes. The inverse current was lmilliampere at volts. We have been able to successfully rectifyelectrical current at voltages of up to 300 volts with the rectitiersconstructed as illustrated in Fig. 4 The reverse current increasesslowly with the temperature rise above room temperature. At 100 volts,the silicon diode rectifier had au inverse current of approximately 10milliamperes at C.

Rectifiers constructed in accordance with the present invention havefunctioned satisfactorily for long periods of time at temperatures of200 C. without difficulty and with outstanding efficiency for suchelevated temperatures.

For rectifiers to be employed in radio, television and other electronicdevices requiring only small rectified currents, such as from 1milliampere to 100 milliamperes, the rectifier diode assembly 10 can beplaced in a glass or ceramic receptacle with two bushings, such as 114of Fig. 4, sealed to the glass walls to pass the conductors to theinterior, the end contact 12 being soldered to the glass walls by ametal coating such as a platinum glaze on the glass Wall in order todissipate heat to the glass wall.

It will be understood that the above description and drawings areillustrative and not limiting.

We claim as our invention:

l. In a semiconductor device, in combination, a semiconducting membercomprising silicon, a contact member having a surface closely conformingto and disposed adjacent to a surface of the semiconducting member, thecontact member composed of a metal selected from the group consisting ofmolybdenum, tungsten and base alloys thereof having a coefficient ofthermal expansion corresponding closely to that of silicon, and a fusedlayer disposed between and bonded to said adjacent conforming surfaces,the fused layer consisting of an alloy of silver and at least oneelement of group IV-B of the periodic table selected from the groupconsisting of tin, silicon, germanium and lead, the alloy composed of atleast 5% silver, the balance not exceeding 90% by weight of tin, notexceeding 20% by weight of silicon, not exceeding 50% by weight ofgermanium, not exceeding 95% by weight of lead.

2. A semiconductor rectifier comprising, in combination, a silicon waferhaving a surface, a first contact member having a surface closelyconforming to and disposed adjacent to the said surface of the siliconwafer, the contact member composed of a metal selected from the groupconsisting of molybdenum, tungsten and base alloys thereof having acoe'icient of thermal expansion corresponding closely to that of thesilicon wafer, and a fused layer disposed between and bonded to saidadjacent conforming surfaces, the fused layer consisting of an alloycomposed of silver and at least one element selected from the groupconsisting of tin, silicon, germanium and lead, the alloy composed of atleast 5% by weight silver, the alloy not exceeding 95% tin, notexceeding 20% silicon, not exceeding 50% germanium and not exceeding 95%lead, the fused alloy having a melting point of between 225 C. and 925C., and a second contact member of the metal employed for the firstcontact bonded to another surface of the silicon Wafer.

3. A semiconductor diode comprising, in combination, an end contactmember with at least one fiat surface and composed of a metal selectedfrom the group consisting of molybdenum, tungsten and base alloysthereof having a coeiiicient of thermal expansion corresponding closelyto that of silicon, a wafer of silicon having N-type conductivity andhaving two fiat surfaces, superimposed on the tiat surface of the endcontact member, a fused layer disposed between and bonded to the fiatsuperimposed surfaces of the end Contact member, a fused layer disposedbetween and bonded to the at superimposed surfaces of the end contactmember and the silicon wafer, the fused layer consisting of an alloy ofsilver and at least one element selected from the group consisting oftin, silicon, germanium and lead, the alloy composed of at least 10% byweight of silver, and the alloy not exceeding 90% tin, not exceeding 16%silicon, not exceeding 50% lead and not exceeding 30% by weight ofgermanium, a second contact member of the same metal as the end contactmember, the second contact member having a flat surface, a layer of aP-type material interposed between the second contact member and theother flat surface of the silicon wafer, the layer of P-type materialdisposed bonding the second contact member to the silicon wafer, thethin layer of P-type material selected from the group consisting ofaluminum and alloys of aluminum with at least one element selected fromthe group consisting of silicon, germanium, gallium, and indium, theP-type material diffused into the silicon to convert the adjacentsilicon to the P-type thereby providing a P-N junction.

4. A semiconductor rectifier diode comprising, in combination, a flatend contact member of molybdenum, a thin fragile flat wafer of siliconhaving N-type conductivity superposed on the end contact member, a fusedlayer disposed between and bonding the silicon wafer to the end contact,the fused layer comprising an alloy of from 2% to 5% germanium, a smallamount of dissolved silicon and the balance being silver, a secondcontact member with a at surface superposed on the other surface of thesilicon wafer, a thin layer of fused aluminum material selected from thegroup consisting of aluminum and alloys of aluminum with at least oneelement selected from the group consisting of silicon, germanium, indiumand gallium, disposed between and bonding the silicon wafer to thesecond contact member, the aluminum material from the fused layerpenetrating into and producing an adjacent layer in the silicon waferwith P-type conductivity.

5. In an air-cooled semiconductor rectifier device, in combination, asealed metallic casing, the casing including insulating means passing anelectrical conductor through the walls of the casing, a semiconductorP-N junction rectifier soldered to the Wall of the casing so as toconvey rapidly to the casing heat developed during operation of therectifier, the rectifier comprising an end contact member which has onesurface soldered to the metallic casing, the end contact member composedof a metal selected from the group consisting of molybdenum, tungsten,and base alloys thereof having a coefficient of thermal expansioncorresponding closely to that of silicon, a silicon wafer having N-typeconductivity superposed on another surface of the end contact member, afused layer disposed between and bonded to the said another surface ofthe end contact and a surface of the silicon Wafer, the fused layerconsisting of an alloy composed of silver an-d at least one elementselected from the group consisting of tin, silicon, germanium and lead,the alloy composed of at least 5% by weight of silver' and the balancenot eX- ceeding 95% by weight of tin, not exceeding 20% by weight ofsilicon, not exceeding 50% by weight of germanium and not exceeding 95%by weight of lead, an upper contact member of the alloy employed for theend contact member disposed immediately above the silicon wafer, a thinfused layer of P-type material disposed between and bonded to anothersurface of the silicon wafer and to the upper contact member, the P-typealuminum material selected from the group consisting of aluminum andalloys of aluminum with at least one element selected from the groupconsisting of silicon, germanium, indium, and gallium, the P-typealuminum material being diffused into the silicon wafer to convert theadjacent silicon to P-type conductivity, thereby producing a P-Njunction, electrical conductors connected to the upper contact member,and radiator means connected to the casing to dissipate to theatmosphere heat imparted by the rectifier to the metallic casing.

6. The rectifier device of claim 5, wherein the radiator means comprisesa body of metal having a cavity within which the metallic casing fitsclosely and is soldered therein to provide good metal-to-metal contact.

7. ln an air-cooled semiconductor rectifier device, in combination, asealed metallic casing, the casing including insulating means passing anelectrical conductor through the walls of the casing, a semiconductorP-N junction rectifier in contact with the wall of the casing so as toconvey rapidly to the casing heat developed during operation of therectifier, the rectifier comprising an end contact member which has onesurface soldered to the metallic casing, the end contact member composedof muiybdenuin, a silicon wafer having N-type conductivity superposed onanother surface of the end Contact member, a fused layer disposedbetween and bonded to the said another surface of the end contact and asurface of the silicon wafer, the fused layer consisting of an alloycomposed of silver, silicon and germanium, the germanium comprisingbetween 2% and 5% by weight, a small amount of silicon and the balancebeing silver, an upper contact member of molybdenum, a thin fused layerof P-type material disposed between and bonded to another surface of thesilicon wafer and to the upper contact member, the P-type aluminummaterial selected from the group consisting of aluminum and alloys ofaluminum with at least one element selected from the group consisting ofsilicon, germanium. indium and galliurn, the P-type aluminum materialbeing diffused into the silicon wafer to convert adjacent silicon toP-type conductivity, thereby producing a P-N junction, electricalconductors connected to the upper contact member, and radiator meansconnected to the casing to dissipate to the atmosphere heat imparted bythe rectifier to the metallic casing.

S. ln the process of producing a semiconductor rectifier device, thesteps comprising heating to a maximum temperature of between 850 C. and925 C. in a vacuum, a superimposed assembly of (l) an end contact membercomposed of a metal from the group consisting of molybdenum, tungstenand `base alloys thereof having a coefficient of thermal expansioncorresponding closely to that of silicon, (2) a thin layer of athickness of the order of from l to 2 mils of an alloy of silver and atleast one element selected from the group consisting of tin, silicon,germanium and lead, the alloy composed of at least 5% by weight ofsilver and the balance not eX- ceeding by weight of tin, not exceeding20% by weight of silicon, not exceeding 50% by weight of germanium andnot exceeding 95% by Weight of lead, (3) a wafer of a thickness of theorder of l0 mils of N-type silicon, (4) a thin layer of the order offrom l to 2 mils of aluminum material selected from the group consistingof aluminum and alloys of aluminum with at least one element selectedfrom the group consisting of silicon, germanium, indium and gallium,capable of conferring P-type conductivity to silicon, and (5) anothercontact of the same metal as the end contact, the assembly being underlight pressure, whereby the thin layer of the silver alloy fuses and thcthin layer of aluminum material fuses to the upper contact and thesilicon wafer, and diffuses into the silicon wafer to convert theadjacent silicon into P-type silicon, and then cooling the assembly,thereby producing a bonded unitary rectifier member having a P-Njunction.

References Cited in the le of this patent UNITED STATES PATENTS2,662,997 Christensen Dec. 15, 1953 2,689,930 Hall Sept. 2l, 19542,701,326 Pfann et al. Feb. l, 1955

1. IN A SEMICONDUCTOR DEVICE, IN COMBINATION, A SEMICONDUCTING MEMBERCOMPRISING SILICON, CONTACT MEMBER HAVING A SURFACE CLOSELY CONFORMINGTO AND DISPOSED ADJACENT TO A SURFACE OF THE SEMICONDUCTING MEMBER, THECONTACT MEMBER COMPOSED OF A METAL SELECTED FROM THE GROUP CONSISTING OFMOLYBDENUM, TUNGSTEN AND BASE ALLOYS THEREOF HAVING A COEFFICIENT OFTHERMAL EXPANSION CORRESPONDING CLOSELY TO THAT OF SILICON, AND A FUSEDLAYER DISPOSED BETWEEN AND BONDED TO SAID ADJACENT CONFORMING SURFACES,THE FUSED LAYER CONSISTING OF AN ALLOY OF SILVER AND AT LEAST ONEELEMENT OF GROUP IV-B OF THE PERIODIC TABLE SELECTED FROM THE GROUPCONSISTING OF TIN, SILICON, GERMANIUM AND LEAD, THE ALLOY COMPOSED OF ATLEAST 5% SILVER, THE BALANCE NOT EXCEEDING 90% BY WEIGHT OF TIN, NOTEXCEEDING 20% BY WEIGHT OF SILICON, NOT EXCEEDING 50% BY WEIGHT OFGERMANIUM, NOT EXCEEDING 95% BY WEIGHT OF LEAD.